当我们在Django中添加模型字段时,我们通常这样写:
models.CharField(max_length=100, null=True, blank=True)
ForeignKey, DecimalField等也是如此。两者的基本区别是什么:
null = True只
空白= True只
null=True, blank=True
对于不同的(CharField, ForeignKey, ManyToManyField, DateTimeField)字段?使用选项1、2或3的优点/缺点是什么?
正如Django Model Field reference中所说:Link
Field options
The following arguments are available to all field types. All are optional.
null
Field.null If True, Django will store empty values as NULL in the database. Default is False.
Avoid using null on string-based fields such as CharField and
TextField because empty string values will always be stored as empty
strings, not as NULL. If a string-based field has null=True, that
means it has two possible values for "no data": NULL, and the empty
string. In most cases, it’s redundant to have two possible values for
"no data"; the Django convention is to use the empty string, not
NULL.
For both string-based and non-string-based fields, you will also need
to set blank=True if you wish to permit empty values in forms, as
the null parameter only affects database storage (see blank).
Note
When using the Oracle database backend, the value NULL will be stored to denote the empty string regardless of this attribute
blank
Field.blank
If True, the field is allowed to be blank. Default is False.
Note that this is different than null. null is purely
database-related, whereas blank is validation-related. If a field
has blank=True, form validation will allow entry of an empty value.
If a field has blank=False, the field will be required.
null=True在数据库中的列上设置null(而不是NOT null)。Django字段类型(如DateTimeField或ForeignKey)的空白值将在DB中存储为NULL。
Blank确定表单中是否需要该字段。这包括管理表单和自定义表单。如果blank=True,则该字段将不是必需的,而如果为False,则该字段不能为空。
这两者的组合非常常见,因为通常情况下,如果您要允许表单中的字段为空,您还需要数据库允许该字段为NULL值。例外是CharFields和TextFields,它们在Django中永远不会被保存为NULL。空白值作为空字符串存储在DB中(")。
举几个例子:
models.DateTimeField(blank=True) # raises IntegrityError if blank
models.DateTimeField(null=True) # NULL allowed, but must be filled out in a form
显然,使用这两个选项在逻辑上没有意义(尽管如果您希望一个字段在表单中始终是必需的,则可能存在null=True, blank=False的用例,当通过shell之类的东西处理对象时是可选的)。
models.CharField(blank=True) # No problem, blank is stored as ''
models.CharField(null=True) # NULL allowed, but will never be set as NULL
CHAR和TEXT类型永远不会被Django保存为NULL,所以NULL =True是不必要的。但是,您可以手动将其中一个字段设置为None以强制将其设置为NULL。如果你有一个场景,在那里可能是必要的,你仍然应该包括null=True。